The present invention relates to a polyethylene nonwoven fabric and a laminate comprising the same. More particularly, the present invention relates to a polyethylene nonwoven fabric having a small diameter of fibers making up the nonwoven fabric and good formation as well as to a nonwoven fabric laminate having excellent softness, water impermeability and interlaminar bond properties obtained using the polyethylene nonwoven fabric.
It is known that nonwoven fabrics prepared using polyethylene fibers are soft and comfortable to the touch. However, due to general difficulty in spinning of polyethylene fibers, polyethylene nonwoven fabrics prepared by the conventional meltblowing process have a larger fiber diameter and poor formation. In order to reduce the fiber diameter of polyethylene fibers, it was necessary to elevate a spinning temperature but in this case, gel formation tends to occur sometimes.
It was then attempted to form fibers using a polyethylene of a lower molecular weight than that of a general-purpose polyethylene resin, e.g., a polyethylene wax. However, it was difficult to produce a web continuously, since such a low molecular weight polyethylene had a poor single yarn strength and caused serious fuzzing though its spinnability was good.
Japanese Patent Laid-Open Publication No. 63-165511 discloses a method for preparing polyethylene fibers which comprises blending a linear low density polyethylene having a melt index of less than 40 with a low molecular weight polyethylene having a melt index of 40 or more and one or two members selected from liquid paraffin and then melt-extruding the blend at a particular temperature, thereby to allegedly obtain nonwoven fabrics having a fine fiber in size and soft touch. However, the fiber diameter of the nonwoven fabric produced by the method is at best up to 2 denier (approximately 18 xcexcm), which is not fine enough.
Therefore, an object of the present invention is to provide a polyethylene nonwoven fabric, which has a very small diameter of fibers and excellent uniformity. Another object of the present invention is to provide a nonwoven fabric laminate having excellent softness, water impermeability and interlaminar bond properties, using the polyethylene nonwoven fabric.
To achieve the foregoing objects, the present invention provides a polyethylene nonwoven fabric produced from a resin composition comprising a polyethylene (A) and a polyethylene wax (B) by the meltblowing process.
To achieve the foregoing objects, the present invention also provides a nonwoven fabric laminate comprising a plurality of nonwoven fabric layers, in which at least one of the nonwoven fabric layers is a layer comprising the above-mentioned polyethylene nonwoven fabric.
The polyethylene nonwoven fabric of the invention (hereinafter referred to as xe2x80x9cnonwoven fabric of the inventionxe2x80x9d) and the nonwoven fabric laminate comprising the same will be described below in more detail.
The nonwoven fabric of the invention contains the fibers comprising a resin composition comprising a polyethylene (A) and a polyethylene wax (B).
The polyethylene (A) in accordance with the present invention which is the essential component for forming the fibers to constitute the nonwoven fabric of the invention includes a homopolymer of ethylene and a copolymer of ethylene and other monomer(s). The copolymer may be a random copolymer or a block copolymer. The other monomers include xcex1-olefins having 3 to 20 carbon atoms such as propylene, 1-butene, 1-pentene, 1-hexene, 4-metyl-1-pentene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene and 1-eicosene. Specific examples of this polyethylene are copolymers of ethylene with xcex1-olefins such as 4-methyl-1-pentene and 1-hexene. In the copolymers, the ethylene monomeric unit content is generally at least 80 mol %, preferably in the range of 90 to 99.5 mol % as determined by 13C-NMR.
As the fibers making up the nonwoven fabric of the invention, these polyethylenes may be employed alone or in combination of two or more.
According to the present invention, the polyethylene (A) has preferably a weight-average molecular weight (Mw) in the range of 21,000 to 45,000, more preferably 23,000 to 40,000, in terms of spinnability and kneading compatibility with the polyethylene wax (B). In the present invention, the weight-average molecular weight (Mw) was determined by gel permeation chromatography under the following conditions.
Apparatus Used
Measurement apparatus: Gel permeation chromatograph (manufactured by Waters Inc., Model 150-C)
Analyzer: System controller (manufactured by Toso Corporation, Model SC-8010)
Detector: Differential refractometer
Conditions for Measurement
Column: TSK gel GMH6-HTxc3x971+TSK gel GMH6-HTLxc3x971 (7.8 mmIDxc3x9760 mmL, manufactured by Toso Corporation)
Moving phase: o-Dichlorobenzene (hereinafter abbreviated as ODCB)
Stabilizer for the moving phase: 2,6-di-tert-butyl-p-cresol (5 g/20 kg-ODCB)
Column temperature: 140xc2x0 C.
Flow rate: 1.0 ml/min
Feeding volume: 500 xcexcl
Concentration of a sample measured: 30 mg/20 ml-ODCB
Concentration of the standard sample: 15 mg/20 ml-ODCB
Molecular weight calibration: monodispersed 16 polystyrenes (manufactured by Toso Corporation)
It is preferred that the polyethylene (A) has a density preferably in the range of 0.890 to 0.970 g/cm3, more preferably 0.910 to 0.960 g/cm3, most preferably 0.930 to 0.955 g/cm3. In the present invention, the density of the polyethylene (A) is determined by means of a density gradient tube using a strand, which has been obtained at the time of measurement of a melt flow rate (MFR) at 190xc2x0 C. under a load of 2.16 kg and which is treated by heating at 120xc2x0 C. for 1 hour and slowly cooling to room temperature over 1 hour.
The melt flow rate (MFR) of the polyethylene (A), which has been determined in accordance with ASTM D1238 under the conditions of a temperature of 190xc2x0 C. and a load of 2.16 kg is in the range of generally 15 to 250 g/10 mins., preferably 20 to 200 g/10 mins., more preferably 30 to 200 g/10 mins.
The polyethylene wax (B) which is also the essential component of the fibers making up the nonwoven fabric in accordance with the present invention includes a homopolymer of ethylene and a copolymer of ethylene with other polymerizable monomer(s). As the polymerizable monomers there are the same monomers given as xcex1-olefins for the polyethylene (A). Where a copolymer is used as the polyethylene wax (B), the ethylene monomer unit is contained in the range of generally 80 mol % or more, preferably 90 to 99.5 mol % (as determined by by 13C-NMR).
The softening point of the polyethylene wax (B) is preferably between 110 and 145xc2x0 C. In view of spinnability and kneading compatibility with the polyethylene (A), the weight-average molecular weight (Mw) of the polyethylene wax (B) is preferably 15,000 or less, more preferably in the range of 6,000 to 12,000.
The polyethylene wax (B) may be prepared either by polymerization of a low molecular weight polymer conventionally used or by reducing the molecular weight of a high molecular weight polyethylene through thermal degradation. There is no particular restriction how to prepare the polyethylene wax (B).
In the resin composition which forms the fibers making up the polyethylene nonwoven fabric of the invention, a weight ratio of the polyethylene (A) to the polyethylene wax (B), namely, (A)/(B) is preferably in the range of 90/10 to 10/90, more preferably 30/70 to 70/30, most preferably 40/60 to 60/40.
The melt flow rate (MFR) of the resin composition above, which has been determined in accordance with ASTM D1238 under the conditions of a temperature of 190xc2x0 C. and a load of 2.16 kg is in the range of preferably 300 to 600 g/10 mins., more preferably 400 to 550 g/10 mins.
The resin composition of the present invention may further contain, if necessary and desired, optional components such as other polymers, coloring agents, stabilizers, nucleating agents, etc., so long as the purpose of the invention is not impaired. These optional components include various stabilizers such as known heat stabilizers or weatherproof stabilizers, antistatic agents, slip agents, anti-blocking agents, anti-fogging agents, lubricants, dyes, pigments, natural oil, synthetic oil, etc.
Examples of the stabilizers are anti-aging agents such as 2,6-di-t-butyl-4-methylphenol (BHT), etc.; phenol-based antioxidants such as tetrakis[methylene-3-(3,5-di-t-butyl-4-hydoxyphenyl)propionate]methane, xcex2-(3,5-di-t-butyl-4-hydroxyphenyl) propionic acid alkyl esters, 2,2xe2x80x2-oxamidobis[ethyl-3-(3,5-di-t-butyl-4-hydroxyphenyl). propionate, Irganox 1010 (trademark, hindered phenol type antioxidant), etc.; fatty acid metal salts such as zinc stearate, calcium stearate, calcium 1,2-hydroxystearate, etc.; polyvalent alcohol fatty acid esters such as glycerine monosterate, glycerine distearate, pentaerythritol monostearate, pentaerythritol distearate, pentaerythritol tristearate, etc. These stabilizers may also be used in combination.
The resin composition may further contain fillers such as silica, diatomaceous earth, alumina, titanium oxide, magnesium oxide, pumice powders, pumice balloon, aluminum hydroxide, magnesium hydroxide, basic magnesium carbonate, dolomite, calcium sulfate, potassium titanate, barium sulfate, calcium sulfite, talc, clay, mica, asbestos, calcium silicate, montmorillonite, bentonite, graphite, aluminum powders, molybdenum sulfide, etc.
The polyethylene (A) and the polyethylene wax (B) can be blended with these optional components optionally used depending upon necessity, by means of known techniques.
The nonwoven fabric of the invention can be produced by the meltblowing process which comprises melting and kneading a resin composition comprising the polyethylene (A), polyethylene wax (B) and other optional components, with a extruding machine; extruding the molten composition through a spinneret with spinning nozzles and simultaneously blowing the fiber with a air jet flow of high velocity and high temperature from the periphery of the nozzle to form a web deposit on a collection device in a predetermined thickness as a self-adhesive fine fiber. If necessary and desired, the formed web is then subjected to an entangling processing.
Examples of the entangling processing include a method in which the resulting fibers are bonded by heat embossing using embossing press rolls, a method in which the fibers are bonded by supersonic waves, a method in which the fibers are entangled using a water jet, a method in which the fibers are bonded by passing hot air through the fibers, and a method in which the fibers are entangled with a needle punching. These methods are appropriately chosen and employed to entangle the fibers obtained.
The fineness of the fiber to make up the nonwoven fabric of the invention is preferably not greater than 5 xcexcm in view of uniformity of the nonwoven fabric, more preferably 3 xcexcm or less because a better water impermeability can be obtained.
The present invention further provides a nonwoven fabric laminate comprising a plurality of nonwoven fabric layers, at least one of which is a nonwoven fabric layer comprising the polyethylene nonwoven fabric described above.
In the nonwoven fabric laminate, at least one of the nonwoven fabric layers is made up with the polyethylene nonwoven fabric having the resin composition comprising the polyethylene (A) and the polyethylene wax (B), in order to impart softness, water impermeability (a property showing high water pressure resistance), uniformity and cloth-like appearance and hand to the laminate. The nonwoven fabric laminate of such a multilayered structure may have other additional nonwoven fabrics or films, in addition to the polyethylene nonwoven fabric. The other nonwoven fabrics may be those obtained other than those produced by the meltblowing process, e.g., a dry-spun nonwoven fabric, wet-spun nonwoven fabric or a spunbonded nonwoven fabric. The nonwoven fabric laminate can be a laminate of the polyethylene nonwoven fabric and a film.
Referring to the layer configuration, preferred is such a layer structure comprising at least one spunbonded nonwoven fabric layer and at least one meltblown nonwoven fabric layer, one surface or both surfaces of which are a spunbonded nonwoven fabric layer, because the laminate thus constructed gives excellent wear resistance and resistance to fuzzing.
As the resin which forms the spunbonded nonwoven fabric used in the nonwoven fabric laminate of the present invention, there are polyolef in compositions comprising an ethylene-based polymer such as polyethylene, a propylene-based polymer such as polypropylene, a polyolef in composition containing at least an ethylene-based polymer, etc., from an aspect of adhesion to the polyethylene nonwoven fabric.
Of these resins, particularly preferred examples of the spunbonded nonwoven fabrics include a spunbonded nonwoven fabric made up of an ethylene-based polymer and a spunbonded nonwoven fabric made up of a conjugate fiber which comprises (a) a propylene-based polymer and (b) an ethylene-based polymer and in which the weight ratio of (a) to (b) [(a)/(b)] is in the range of preferably 5/95 to 70/30, more preferably 5/95 to 50/50, much more preferably 10/90 to 40/60, most preferably 10/90 to 20/80 and (b) forms at least a part of the fiber surface. Where the ratio of the propylene-based polymer (a) to the ethylene-based polymer (b) in the conjugate fiber is within the preferred range above, the strength and softness of the nonwoven fabric is well balanced.
Preferred examples of the conjugate fibers include (1) a concentric core-sheath type conjugate fiber made up with the core part comprising the propylene-based polymer (a) and the sheath part comprising the ethylene-based polymer (b); (2) an eccentric core-sheath type conjugate fiber made up with the core part comprising the propylene-based polymer (a) and the sheath part comprising the ethylene-based polymer (b); and (3) a side-by-side type conjugate fiber made up with the propylene-based polymer (a) and ethylene-based polymer (b). Of these conjugate fibers, (2) the eccentric core-sheath type conjugate fiber and (3) the side-by-side type conjugate fiber become crimped and hence, are more preferred in terms of softness.
As the propylene-based polymer (a) which makes up the conjugate fibers, preferred are a propylene hompolymer and a propylene-ethylene random copolymer, having the ethylene monomeric unit content of 0 to 5 mol %. It is desired from a viewpoint of spinnability that the propylene-based polymer (a) used in the conjugate fibers has the melt flow rate (MFR: as 5 determined under a load of 2.16 kg at 230xc2x0 C. in accordance with ASTM D1238) in the range of preferably 20 to 100 g/10 minutes, more preferably 30 to 70 g/10 minutes. Furthermore, the Mw/Mn (Mw: weight-average molecular weight; Mn: number-average molecular weight, determined as in the Mw measurement described above) is preferably in the range of 2 to 4 from the viewpoint of spinnability. The term spinnability as used herein means the property that the resin can be spun stably without involving breakage of the filament when the nonwoven fabric is prepared by spinning the melted resin through a spinneret according to the spunbonding process.
Examples of the ethylene-based polymer (b) forming the conjugate fiber include an ethylene hompolymer (manufactured either by the low-pressure process or by the high-pressure process) and a random copolymer of ethylene and an xcex1-olefin such as propylene, 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene. It is preferred from the viewpoint of spinnability that these ethylene-based polymers (b) have a density in the range of 0.87 to 0.98 g/cm3, preferably 0.880 to 0.970 g/cm3, more preferably 0.900 to 0.950 g/cm3. Further in view of spinnability, the MFR (as determined under a load of 2.16 kg at 190xc2x0 C. in accordance with ADTM D1238) is preferably in the range of 20 to 60 g/10 minutes and the Mw/Mn is preferably in the range of 1.5 to 4, more preferably 2 to 4. As the ethylene-based polymer (b), an ethylene homopolymer whose density, MFR and Mw/Mn are within the ranges described above is preferred from the viewpoint of the spinnability and softness of the spunbonded nonwoven fabric obtained using such an ethylene homopolymer.
The nonwoven fabric comprising the conjugate fiber described above is excellent in softness as compared to conventional polypropylene-made nonwoven fabrics, because most or all of the conjugate fiber surfaces forming the nonwoven fabric is made up of the ethylene-based polymer (b). Further when the conjugate fiber making up the nonwoven fabric is a crimped fiber, the softness is more improved.
According to the present invention, the ethylene-based polymer (b) may further contain a slip agent such as oleic amide, erucic amide and stearic amide in a ratio of 0.1 to 0.5 wt %. When the slip agent is added to the ethylene-based polymer, the resulting spunbonded nonwoven fabric shows an excellent resistance to fuzzing. In the present invention, the slip agent may also be added to the propylene-based polymer (a).
Further in the present invention, other polymers, coloring agents, heat stabilizers, nucleating agents, etc. may be added, if necessary and desired, to the propylene-based polymer (a) and/or the ethylene-based polymer (b) to the extent that the purpose of the invention is not impaired.
The spunbonded nonwoven fabric comprising the conjugate fiber can be produced by known methods. For example, after the weight ratio of the propylene-based polymer (a) to the ethylene-based polymer (b) is set to meet the range of 5/95 to 70/30, filaments of the conjugate fiber are spun by the two-extruder melting and spinning method which comprises melting each resin with an extruder and discharging each molten resin through a spinneret with spinning nozzles designed to form a desired composite structure. The filaments thus spun are cooled with a cooling fluid and the filaments are then given tension by means of stretching air to achieve a desired fineness. Thereafter the spun filaments are collected on a collection belt until the filaments are allowed to deposit in a predetermined thickness. The collected filaments are then subjected to the processing of tangling to produce the spunbonded nonwoven fabric. The tangling is processed by similar methods as applied to the meltblown nonwoven fabric. Of these methods, the heat embossing processing is preferably applied to the filaments. In the case of applying the heat embossing process, the embossing area percentage may appropriately be determined but preferably is in the range of 5 to 30%.
The diameter of the fiber that makes up this spunbonded nonwoven fabric is generally about 5 xcexcm to about 30 xcexcm (approximately 0.2 to 7 deniers), preferably approximately 10 to 20 xcexcm.
The use of the spunbonded nonwoven fabric described above provides excellent bond strength when the meltblown nonwoven fabric is bonded to the spunbonded nonwoven fabric by heat embossing.
The nonwoven fabric laminate in accordance with the present invention preferably comprises at least one spunbonded nonwoven fabric layer and at least one meltblown nonwoven fabric layer. The layer configuration is not particularly limited so long as at least one of the surface layers is made up with a spunbonded nonwoven fabric layer. However, the laminate preferably takes a layer configuration of a spunbonded nonwoven fabric layer/a meltblown nonwoven fabric layer or, a spunbonded nonwoven fabric layer/a meltblown nonwoven fabric layer/a spunbonded nonwoven fabric layer.
The basis weight of the nonwoven fabric laminate of the present invention may be appropriately chosen depending upon applications of the nonwoven fabric laminate, quality, economics, etc. required for the laminate. In general, the basis weight of the nonwoven fabric laminate is approximately 7 to 50 g/m2, preferably approximately 10 to 30 g/m2.
Any method may be used for manufacturing the nonwoven fabric laminate of the present invention and there is no particular limitation to the method so long as the laminate can be formed into an integral form. Where the laminate is formed using the spunbonded nonwoven fabric and the meltblown nonwoven fabric, any one of the following methods can be applied to produce the laminate: (1) a method which comprises depositing the meltblown fiber formed by the meltblowing process directly onto the spunbonded nonwoven fabric and then thermally bonding the formed meltblown nonwoven fabric to the spunbonded nonwoven fabric; (2) a method which comprises depositing the meltblown fiber formed by the meltblowing process directly onto the spunbonded nonwoven fabric (i) to form the meltblown nonwoven fabric, depositing the spunbonded fiber formed by the spunbonding process directly onto the aforesaid meltblown nonwoven fabric to form the spunbonded nonwoven fabric (ii), and then thermally bonding the spunbonded nonwoven fabric (i) to the meltblown nonwoven fabric and the spunbonded nonwoven fabric (ii); (3) a method which comprises putting the spunbonded nonwoven fabric and the meltblown nonwoven fabric on top of each other and bonding by heating the two fabrics under pressure; and (4) a method which comprises bonding the spunbonded nonwoven fabric to the meltblown nonwoven fabric using an adhesive such as a hot melt adhesive, a solvent-based adhesive, etc.
The nonwoven fabric laminate of the present invention is excellent in interlaminar bond properties since the meltblown nonwoven fabric is made up with the resin composition comprising the polyethylene (A) and the polyethylene wax (B). Therefore, the nonwoven fabric laminate provides a satisfactory bond strength even when adhered by heat fusion such as a heat embossing processing to the spunbonded nonwoven fabric made up with the conjugate fiber comprising the propylene-based polymer (a) and the ethylene-based polymer (b).
For bonding nonwoven fabrics to each other by heat fusion, there are applicable methods in which the entire contact surfaces of the respective nonwoven fabrics are bonded by heat and in which part of the contact surfaces is bonded by heat. According to the present invention, the method of bonding part of the contact surfaces of the respective nonwoven fabrics by heat is preferably used. In this case, the bonded area (which corresponds to the area impressed by the embossing roll) is preferably 5 to 35%, more preferably 10 to 30%, of the contact surface. Where the bonded area is within the range above, the nonwoven fabric laminate has a well balanced property between bond strength and softness.
Examples of the hot melt adhesive which is employed to bond the spunbonded nonwoven fabric to the meltblown nonwoven fabric with an adhesive include resin-based adhesives such as vinyl acetate and polyvinyl alcohol, and rubber-based adhesives such as styrene-butadiene and styrene-isoprene. Examples of the solvent-based adhesive used for the same purpose include rubber-based adhesives such as styrene-butadiene, styrene-isoprene and urethane, and organic solvent-based and aqueous emulsion type adhesives which are based on resins such as vinyl acetate and vinyl chloride. Of these adhesives, the rubber-based hot melt adhesives such as styrene-isoprene and styrene-butadiene are preferred in that these adhesives do not impair the characteristic hand of the spunbonded nonwoven fabric.
The nonwoven fabric laminate of the present invention thus obtained has good uniformity and excellent gas permeability, water impermeability and softness. The nonwoven fabric laminate also has excellent wear resistance and resistance to fuzzing, because the surface of one or both side of the laminate is formed of the spunbonded nonwoven fabric.
The soft nonwoven fabric laminate of the present invention has the KOSHI value, that is an index of softness, of not more than 10 in general, preferably not ore than 9.5. The nonwoven fabric laminate of the invention also has the water impermeability of 60 mmAq or more generally, preferably 90 mmAq or more.
The nonwoven fabric and nonwoven fabric laminate of the present invention described above are applied to a wide variety of sanitary goods, household materials, industrial materials and medical materials. Especially because of their excellent softness, gas permeability and water impermeability, the nonwoven fabric laminate of the invention is used advantageously for a base material for sanitary and packaging materials. More specifically, the nonwoven fabric laminate of the invention is suitably used as substrates of disposable diapers, sanitary napkins, poultice materials, etc. and as a material for bed covers, etc., and as compact disc bags, food packaging materials, clothing covers, etc. for packaging use.